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61 lines
1.8 KiB
Markdown
61 lines
1.8 KiB
Markdown
# Propagating Errors with `?`
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The try-operator `?` is used to return errors to the caller. It lets you turn
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the common
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```rust,ignore
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match some_expression {
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Ok(value) => value,
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Err(err) => return Err(err),
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}
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```
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into the much simpler
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```rust,ignore
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some_expression?
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```
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We can use this to simplify our error handling code:
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```rust,editable
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use std::{fs, io};
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use std::io::Read;
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fn read_username(path: &str) -> Result<String, io::Error> {
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let username_file_result = fs::File::open(path);
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let mut username_file = match username_file_result {
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Ok(file) => file,
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Err(err) => return Err(err),
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};
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let mut username = String::new();
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match username_file.read_to_string(&mut username) {
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Ok(_) => Ok(username),
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Err(err) => Err(err),
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}
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}
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fn main() {
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//fs::write("config.dat", "alice").unwrap();
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let username = read_username("config.dat");
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println!("username or error: {username:?}");
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}
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```
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<details>
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Key points:
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* The `username` variable can be either `Ok(string)` or `Err(error)`.
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* Use the `fs::write` call to test out the different scenarios: no file, empty file, file with username.
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* The return type of the function has to be compatible with the nested functions it calls. For instance,
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a function returning a `Result<T, Err>` can only apply the `?` operator on a function returning a
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`Result<AnyT, Err>`. It cannot apply the `?` operator on a function returning an `Option<AnyT>` or `Result<T, OtherErr>`
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unless `OtherErr` implements `From<Err>`. Reciprocally, a function returning an `Option<T>` can only apply the `?` operator
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on a function returning an `Option<AnyT>`.
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* You can convert incompatible types into one another with the different `Option` and `Result` methods
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such as `Option::ok_or`, `Result::ok`, `Result::err`.
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</details>
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